1. Field of the Invention
The present invention concerns a method for adjustment of a shim device of a magnetic resonance apparatus before an image acquisition in a body region having a volume of interest, wherein a field distribution is measured in an area encompassing the volume of interest produced with a shim device adjusted according to a basic shim data set and wherein a shim data set optimized for the volume of interest is determined by a control device dependent on the field distribution, starting from the basic shim data set and the optimized shim data set is used for adjustment of the shim device.
2. Description of the Prior Art
For image acquisition in magnetic resonance apparatuses, the homogeneity of the magnetic field in the region of interest is of great importance. Small deviations of the homogeneity can lead to large deviations of the frequency distribution, such that qualitatively substandard images are acquired.
Shim devices are known in order to improve the homogeneity in volumes of interest for an image acquisition. When a magnetic resonance apparatus is installed at its installation site, fields or field sources present in the environment can limit the built-in homogeneity of the magnetic field, in particular around the isocenter. Upon installation and start-up of a magnetic resonance apparatus, the shim device is therefore set (frequently in connection with measurements) such that as optimal a homogeneity as possible is established.
However, the subject from whom image data are to be acquired represents a further inhomogeneity source. When a person to be examined is introduced into the magnetic resonance apparatus, the matter of the body disrupts the homogeneity again. In order to counter this problem, it is known to use readjustable shim devices. In particular, electrical shim coils are known that, controlled with various shim currents, generate various compensation magnetic fields in order to improve the homogeneity.
In order to shim these disruptions of the subject to be examined, given activation of the shim device with the basic shim data set acquired during the installation and start-up of the magnetic resonance apparatus it is typical to first conduct a measurement of the field distribution using the magnetic resonance apparatus itself when the person to be examined was introduced into the patient admission of the magnetic resonance apparatus. Starting from the basic shim data set, an optimized shim data set is determined by a control device dependent on the measured field distribution. Using this optimized shim data set the shim device is then controlled in order to achieve a homogeneity as optimal as possible. Various algorithms are known to determine this optimized shim data set, but certain problems occur in the known methods.
Under some circumstances the employed algorithms do not converge at an optimized shim setting when the values of the basic shim data set are too far removed from the optimal possible values. A further error source is in the measurement of the field distribution since greater inhomogeneities, just like measurement errors, can easily lead to the situation that the algorithm supplies no optimal results.
In the prior art it is known to effect one or more further iterations, meaning to possibly effect further field measurements and to newly apply the algorithm until a user is of the opinion that the last determined shim parameter set is optimal.
For this purpose, it is known to implement a further iteration, meaning to effect a further field measurement and to apply the algorithm again until the user is of the opinion that the optimum is reached. This process is, however, extremely time-consuming and prone to error.